In continuous casting, reaching a high casting speed and therefore attaining an always higher productivity, while still maintaining both the surface and internal quality of the cast product high, is correlated to the optimization of a plurality of technological parameters relating both to the characteristics of the crystallizer and to the equipment connected to it, and also to the casting method.
Said parameters mainly concern the geometric and dimensional characteristics of the crystallizer, the primary cooling system, the lubrication system of the internal walls and the material the crystallizer is made of.
Such parameters affect the capacity of the crystallizer to support the high thermal and mechanical stresses and the wear to which it is subjected, thus in practice determining its operating life in conditions of great efficiency.
It must be considered that in a crystallizer there are, at the same time, thermal, mechanical and metallurgical phenomena which influence its longevity and performance.
A distinction must also be made when comparing the dimensions, since crystallizers for “small” products such as billets, have different problems compared to crystallizers for “big” products such as blooms. The former, especially in high speed applications, are extremely stressed from the thermal-mechanic point of view and typically the need to extend their working life is more keenly felt.
A good crystallizer must ensure a reduced distortion, so as to limit the phenomenon of “negative conicity”, above all in the zone of the meniscus. It must also limit the onset and the spread of cracks on the internal surface. It must be able to limit the maximum temperature reached, for a defined couple of casting speed/dimension of the product.
With regard to the geometric and dimensional characteristics, crystallizers of a known type provide a substantially constant thickness of the walls over the whole length of the crystallizer, in particular in a zone comprised between the external surface of the crystallizer and the cooling holes, also called the cold part.
In particular, it is provided that the thickness of the copper wall is directly proportional to the sizes of the cast product, with a typical value of about one tenth of the side of the product.
Increasing the thickness, the conductive heat resistance also increases, so that, given the same heat flow set and the temperature of the cooling water, the maximum temperature also increases. Beyond a certain temperature, or “softening temperature”, the mechanical properties of the copper show a sudden drop and there is a rapid deterioration of the geometric characteristics and resistance to wear of the crystallizer.
The maximum temperature reached depends on the conductive and convective resistances: the first is univocally determined by the thickness and type of copper, the second by the heat exchange coefficient that is obtained by the cooling fluid flowing inside the walls. It has been shown that the first resistance has a preponderant effect on the second.
For “small” products, with a limited copper thickness, cast at high speeds, the heat flows are very high and the distortions of the crystallizer become considerable, invalidating the internal conicity and consequently the continuity of contact between cast product and internal walls of the crystallizer. The lack of contact is harmful for the cast product since it reduces the heat exchange and may create surface defects, such as depressions and longitudinal cracks, as well as slowing the growth of the solid skin.
Given the above, it has happened that solutions adopted in known crystallizers entail, particularly in the zone around the meniscus, that is, the one subject to the highest temperatures in the casting steps of molten steel, a therm-mechanical conditioning of the tensional and deformative state of the crystallizer, limiting the casting speeds obtainable due to the localized plastic deformation of the crystallizer that causes the reduction in its working life.
Furthermore, due to the heat peak in correspondence with the zone of the meniscus, the temperature is not uniform along the crystallizer, which causes a non-uniform therm-mechanic deformation thereof due to the different thermal dilation of the material, with consequent problems connected to the defects of form that this plastic deformation causes on the cast product and the premature wear of the crystallizer, which reduces its working life.
A further problem is connected to maintaining the crystallizer in conditions of efficiency for long periods before having to resort to maintenance and/or replacement, deriving in particular from localized cracks in the zone of the meniscus caused by tensions and plastic deformation accumulated during the heating cycles.
In the crystallizers currently used it has been impossible to find a satisfying solution to all these problems, and indeed the attempt to solve them has instead led to accentuate others.
The prior art documents JP 61 276749 and US 2006/191661 show crystallizers with localized reductions in section, but these crystallizers do not have cooling channels made in the thickness of the copper walls and therefore the therm-mechanic and deformation behavior, in particular in the zone of the meniscus, is completely different from crystallizers equipped with such internal channels.
US 2004/0069458 describes solutions both with internal cooling channels and with cooling using an external jacket, and also with nozzles that spray cooling liquid against the external walls of the crystallizer. This document provides a reduction in thickness of the walls of the crystallizer starting from the top, and also establishes a fixed percentage ratio (in the order of 10%) between the thickness of the copper wall and the side of the cast product, so that as the size of the cast product varies, the thickness of the copper wall of the crystallizer also varies percentage-wise.
As a result of this approach, especially for “small” products like small-size billets, the therm-mechanic deformations and distortions to which the walls of the crystallizer are subject are particularly high. As stated, this can invalidate the internal conicity and therefore the correct contact between the cast product and the walls of the crystallizer, with a consequent reduction in the copper/steel heat exchange. This entails surface defects of the cast product, slows down the growth of the skin and causes bulging of the billet at exit from the crystallizer. To obviate these phenomena, it is necessary to reduce the casting speed and therefore the overall productivity of the plant.
It should also be noted that in U.S. '458 the reduction in thickness is independent of the presence or absence of the cooling holes, since the presence of the cooling holes passing through the walls of the crystallizer is a simple example, not binding for the purposes of the solution proposed.
The present invention therefore proposes to provide a response to all these problems, seeking a solution that allows, firstly, to increase the working life of the crystallizer in conditions of high casting efficiency, also taking into account the need to keep the internal shape, with its substantially conical development, as unchanged as possible.
Purpose of the present invention is therefore to obtain a crystallizer equipped with internal cooling channels which allows to reach high casting speeds and, at the same time, to achieve a high number of casting cycles, substantially reducing the possible therm-mechanic plastic deformations in the zone of the meniscus, so as to increase the working life of the crystallizer in conditions of high efficiency.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.